Many fundamental issues in the study of human language-the role of early experience, the concept of a "critical period," and the contributions of input versus internal predispositions in language acquisition-are difficult to investigate because most children are, fortunately, exposed to rich, fluent input to their primary language from birth. However, deaf children often acquire signed languages from greatly reduced linguistic input. Our research investigates how well they do this, and how languages that emerge from reduced input develop and change over time. These questions are theoretically significant and also crucial for decisions in the lives of deaf children. We propose to continue 3 lines of work. One line of research studies the development of young sign languages, whose grammars are being built and expanded as they are acquired from reduced input. In one setting we are observing 'home sign' systems of isolated deaf children. In a second setting we are studying a family sign language that has been used and handed down across several generations on a remote fishing island. A second line of research examines the early linguistic structure of American Sign Language and the way the language expanded as it passed through early generations of signers. A third line of research investigates the questions of input and language development in a laboratory paradigm, using a miniature language learning technique with hearing adults and children to investigate how languages are acquired and changed when input is reduced or altered. This research will contribute to our understanding of the relation between input and outcome in language acquisition, and of the mechanisms by which children acquire their native languages. In addition, it will contribute to decisions regarding early language exposure, whether signed or spoken, in deaf education and parent counseling.